1. Field of the Invention
This invention relates to a differential amplifier and to a method of obtaining an imbalance-corrected differential amplifier.
2. Discussion of Prior Art
Differential amplifiers are well known in the prior art. They are based on the so-called long-tailed pair of active three terminal devices. The devices of such a pair have separate input and load connections, but their third terminals are connected together; ie the pair may be a cathode-coupled valve pair, an emitter-coupled bipolar transistor pair or a source-coupled field effect transistor pair. More complex circuits are also known in which a differential amplifier is an input stage of an operational amplifier, and converts two input signals to a single differential signal. In recent years, differential and operational amplifier production has been preferentially based on integrated circuit techniques using complementary metal oxide-silicon (CMOS) technology.
An ideal differential amplifier would have a predetermined output voltage for zero input voltages. It would also have a predetermined output voltage when both input voltages are non-zero but equal; ie the common mode rejection ratio (CMRR) of the amplifier should be infinite. In practice, this ideal is not achieved because of mis-match between component transistors of a differential amplifier.
A differential amplifier has at least one pair of transistors which should ideally be identical. Unfortunately, variations in manufacturing lead to differences between pair members which produce discrepancies between ideal performance and obtained performance.
To improve differential amplifier performance over that obtainable by attention to design and manufacture, it is known to adjust the physical parameters of such amplifiers subsequent to manufacture. One such approach involves incorporating in the amplifier a resistor whose value controls an electrical characteristic of the amplifier. The resistor is made of vaporizable material, and is located in an accessible position in the amplifier. The resistor is partially vaporized by a laser to alter its resistance to a value appropriate to adjust the amplifier's performance. In practice, a circuit requires robust construction and therefore increased cost to tolerate thermal stresses associated with laser vaporization. Moreover, to achieve adequate performance, it may be necessary to employ more than one vaporizable resistor. The process is limited in accuracy to the minimum change in a resistor value obtainable by a single laser pulse, and is time consuming and expensive.
An alternative approach to differential amplifier adjustment is to replace individual resistors requiring adjustment by respective chains of resistors in series, each resistor being bridged by a Zener diode with associated current terminal pad. One or more individual resistors in each chain may be short-circuited by passing a high reverse current through the associated Zener diodes. The current short-circuits the diode junction, and the chain resistance becomes that due to those remaining resistors which are not short-circuited. Conveniently, the resistor values in a chain form a binary doubling sequence, ie R, 2R, . . . 2.sup.n-1 R; seven resistors per chain are needed to achieve resistance adjustment accuracy to 1% of the chain resistance. Moreover, each resistor requires a contact pad. This approach has the disadvantage that part of the circuit is devoted to resistors, contact pads and Zener diodes which are irrelevant to circuit operation after initial circuit adjustment. It is therefore wasteful of integrated circuit material.
It has also been suggested to employ a differential current circuit as a module for insertion in an amplifier to provide current adjustment capability. This is reported by E Sackinger and W Guggenbuhl IEEE J. SC23 1437 (1988), who describe a sub-circuit module suitable for delivering a current of magnitude and polarity required for amplifier adjustment.
An alternative approach has been suggested by L. R. Cartey, IEEE J. SC24 1569 (1989). This involves an amplifier with two input field effect transistors each with a drain load transistor in what is known in the prior art as a "current mirror" arrangement. Carley suggests employing an additional transistor connected in parallel with a drain load transistor and arranged to counteract offset voltage.
All the foregoing prior art techniques for differential amplifier adjustment suffer from the disadvantage that they counteract mis-match by the introduction of asymmetry into the circuit. The traditional long-tailed pair differential amplifier is symmetrical; it has two input circuits which are intended to be identical to one another. The symmetry is lost if resistors and Zener diodes or additional load transistors are used to counteract mis-match. Such prior art techniques allow the amplifier offset voltage to be adjusted and forced to zero for a particular combination of input voltages. However, the average offset voltage over the whole range of amplifier inputs may worsen. That is, the circuit asymmetry degrades the CMRR of the differential amplifier.